Lucas H. Gabrielli

WDM-compatible mode-division multiplexing on a silicon chip

Significant effort in optical-fibre research has been put in recent years into realizing mode-division multiplexing (MDM) in conjunction with wavelength-division multiplexing (WDM) to enable further scaling of the communication bandwidth per fibre. In contrast, almost all integrated photonics operate exclusively in the single-mode regime. MDM is rarely considered for integrated photonics because of the difficulty in coupling selectively to high-order modes, which usually results in high inter-modal crosstalk. Here we show the first microring-based demonstration of on-chip WDM-compatible mode-division multiplexing with low modal crosstalk and loss. Our approach can potentially increase the aggregate data rate by many times for on-chip ultrahigh bandwidth communications.

On-chip transformation optics for multimode waveguide bends

Current optical communication systems rely almost exclusively on multimode fibres for short- and medium-haul transmissions, and are now expanding into the long-haul arena. Ultra-high bandwidth applications are the main drive for this expansion, based on the ability to spatially multiplex data channels in multimode systems. Integrated photonics, on the other hand, although largely responsible for todays telecommunications, continues to operate almost strictly in the single-mode regime. This is because multimode waveguides cannot be compactly routed on-chip without significant inter-mode coupling, which impairs their data rate and prevents the use of modal multiplexing. Here we propose a platform for on-chip multimode devices with minimal inter-mode coupling, opening up the possibilities for integrated multimode optics. Our work combines a novel theoretical approach--large-scale inverse design of transformation optics to maximize performance within fabrication constraints-with unique grayscale-lithography fabrication of an exemplary device: a low-crosstalk multimode waveguide bend.

Integrated Luneburg lens via ultra-strong index gradient on silicon

Gradient index structures are gaining increased importance with the constant development of Transformation Optics and metamaterials. Our ability to fabricate such devices, while limited, has already demonstrated the extensive capabilities of those designs, in the forms of invisibility devices, as well as illusion optics and super-lensing. In this paper we present a low loss, high index contrast lens that is integrated with conventional nanophotonic waveguides to provide improved tolerance in fiber-to-chip optical links for future communication networks. This demonstration represents a positive step in making the extraordinary capabilities of gradient index devices available for integrated optics.

Focusing light in a curved-space

We use transformation optics to demonstrate focusing of light. The lenses are designed and fabricated on silicon with dimensions ranging from 20µm × 20 µm to 5.0 µm × 5.0 µm. The numerical and experimental results show the focusing of light over a broad wavelength range, from 1.15 µm to 1.60 µm.

Transformation optics on a silicon platform

Transformation optics allows the creation of innovative devices; however, its implementation in the optical domain remains challenging. We describe here our process to design and fabricate such devices using silicon as a platform for broad band operation in the optical domain. We discuss the approximations and methods employed to overcome the challenges of using dielectric materials as a platform for transformation optics, such as the anisotropy and gradient refractive index implementation. These encompass conformal and quasi-conformal mappings, and a dithering process to discretize and quantize the continuously inhomogeneous index function. We show examples of devices that we fabricated and tested, including the carpet invisibility cloak, a broad bandwidth light concentrator, and a perfect imaging device, known as Maxwells fish eye lens. Finally, we touch on future directions under investigation to further develop transformation optics based on dielectric materials.

Study of a low-cost trimodal polymer waveguide for interferometric optical biosensors

A novel evanescent wave biosensor based on modal interaction between the fundamental mode and the second order mode is proposed and numerically demonstrated. By taking advantage of their symmetries, it is possible to design a device where only the fundamental and the second order modes can propagate, without excitation of the first order mode. With this selection of modes it is possible to achieve a high sensitivity behavior in the biosensor configuration, due to the strong interaction between the evanescent field and the outer surface as compared to previous evanescent wave-based biosensor designs.

Anisotropy minimization via least squares method for transformation optics

In this work the least squares method is used to reduce anisotropy in transformation optics technique. To apply the least squares method a power series is added on the coordinate transformation functions. The series coefficients were calculated to reduce the deviations in Cauchy-Riemann equations, which, when satisfied, result in both conformal transformations and isotropic media. We also present a mathematical treatment for the special case of transformation optics to design waveguides. To demonstrate the proposed technique a waveguide with a 30° of bend and with a 50% of increase in its output width was designed. The results show that our technique is simultaneously straightforward to be implement and effective in reducing the anisotropy of the transformation for an extremely low value close to zero.

Breakthroughs in Photonics 2013: Advances in Nanoantennas

The field of nanoantennas overlaps several areas of scientific interest, from fundamental physics to commercial technologies. Advances in the understanding of such complex devices hold the promises to novel applications in sensing, telecommunications, optical processing, and security, among others. Here, we review the main advances in the field of nanoantennas in 2013, from single metallic and dielectric devices to nanostructured metasurfaces and phased arrays.

Robustness Optimization of Fiber Index Profiles for Optical Parametric Amplifiers

Optical fibers with index profiles that are optimized for broadband double pumped parametric amplifiers are thoroughly investigated. The analysis includes random variations in the radii of the profile along the fiber length (introduced in the fabrication process). By optimizing the fourth order dispersion parameter and the effective area we obtain fiber designs that provide flat spectral gain over 200 nm with 3.5 W pump lasers and, at the same time, have robust operation wavelength against fluctuations of up to 1% in the radii of the index profile. These fibers show positive and very small fourth order dispersion. Our results indicate that in such low dispersion fibers, the sixth order dispersion parameter must be considered in the optimization algorithm.

Low-loss modified SU-8 waveguides by direct laser writing at 405 nm

In this work we present a fabrication process to obtain a low-loss waveguide in the photo-curable resin SU-8 using direct laser writing at 405 nm wavelength. Polymer-based devices offer low-cost prototype fabrication, fabrication flexibility, reliability, low power consumption and potential for mass production. These characteristics, coupled with its high optical performance and low propagation losses, make it an attractive material for applications related to optical biosensing. Initially, a method to reduce SU-8 viscosity is described to allow film thicknesses of a few hundred nanometers, thus guaranteeing single mode propagation at visible range. This is achieved while also introducing an H-nu 470 photoinitiator, providing the displacement of the absorption peak of the material from 365 nm to 470 nm, thus allowing H-line polymerization and the direct laser writing at wavelengths 405 nm and above. Key material and structure characteristics such as absorbance, transmittance, roughness and chemical composition on the surface are analyzed for both pure and modified SU-8. We observe lower RMS surface roughness in the latter one. In spite of the chemical modification of the material, optical parameters like absorption and refractive index in the wavelength of interest are not affected. Single- and multimode optical waveguides are demonstrated. The sidewall roughness is measured at 5.6 nm, and the propagation loss for the single mode waveguide is 4.4 dB/cm at 633 nm wavelength, providing a high quality and low-cost fabrication platform for optical nano-devices.